The present invention relates to an oscillatory electrodynamic compressor for use in for example small refrigerators or the like, comprising a coaxially arranged gas compression assembly and electromagnetic drive assembly.
In a known construction shown in German Patent publication DT-OS No. 23,17,038 corresponding to U.S. Pat. No. 3,903,438, the device comprises a closed housing and the electromagnetic drive assembly comprises a permanent magnet having concentrically arranged a cylindrical outer pole shoe and an inner pole shoe spaced to provide an air gap therebetween for the production of a generally radial force field. Arranged to freely move within the air gap is a coil, formed about a pot shaped carrier mounted at the end of a hollow plunger which is part of the compression assembly, which also includes a cylindrical member axially spaced from and fixed to the electromagnetic assembly. The plunger has a unidirectional suction valve at one end allowing passage of a fluid from the housing, while the cylinder assembly includes an axial bore supporting the plunger and a compression chamber communicating with it. A unidirectional discharge valve is interposed between the bore and the compression chamber whereby fluid compressed in the cylinder is discharged to the chamber under a given pressure. The pot shaped coil carrier is resiliently floated between a pair of axially opposed compression springs. One compression spring located on the magnet drive side of the carrier bears against one side of the carrier bottom and the inner magnet pole shoe while the other spring located on the compression side bears against the opposite side of the carrier bottom and the compression cylinder member. On application of an alternating current to the pot shaped coil an oscillating motion is set up between the magnet and coil which causes the coil to "plunge" axially into and out of the gap causing the plunger to reciprocate in the cylinder bore, sucking fluid from the housing, into the bore and subsequently discharging the fluid under pressure. The natural frequency of the oscillating system, comprising the springs, plunger, coil, gas spring rate, etc., is adjusted, to be at least approximately the frequency of the current supplied to the coil.
In the aforementioned device the pot shaped carrier is mounted at the extreme end of the plunger so that the coil extends fully between the outer cylinderical pole shoe and the inner pole shoe.
If there is no excitation current the two vibrating support spring maintain the coil in a center rest position in which the coil carrier is located at a distance from the compression cylinder member and particularly from the end of the cylinder bore as well as from the inner pole shoe. Correspondingly, the planes in which the vibrating springs bear against either side of the coil carrier bottom, are also located at a distance from the end of the cylinder which distance is larger than half the vibration amplitude or larger than half the vibration amplitude plus the distance of the two bearing planes from one another.
During the assembly the electromagnetic drive and the compressor must be exactly centered relative to one another in the same way as the coil, the coil carrier and the plunger must be exactly centered relative to one another to achieve a centric motion of the coil in the magnetic field. Due, however, to the design of the coil as a cylinder and due to the radial configuration of the magnetic force field in the annular gap the force exerted on the current carrying coil has an exact axial action according to the vector equation P = Bxi.L. To avoid any transverse forces which increase the friction of the plunger in the cylinder bore and thus result in an increased wear as well as in an increased energy consumption, the coil carrier portion extending into the magnetic field preferably consists completely of nonferromagnetic material so that there are only forces acting according to the above-mentioned equation. Consequently, transverse forces, i.e. forces having a component radially to the longitudinal axis of the entire system, may only be caused by the vibrating springs. Admittedly, hitherto the vibrating springs for the known device have been exactly selected for plane-parallelism of the end faces and for a centric force transmission. Nevertheless the occurrence of transverse forces could not be completely eliminated due to the anavoidable manufacturing tolerances and the design of such helical springs. These transverse forces generate a tilting moment acting on the plunger which, at a given transverse force, increases the longer the effective lever arm becomes. The lever arm is a function of the distance of the points of impact of the force on the coil carrier from the end of the cylinder bore. Consequently, the tilting moment attains its maximum value when the coil is extended to a maximum depth into the electromagnet. A plunger compressor is also known from German patent publication DT-OS No. 23 18 711 wherein only one single vibrating spring is provided which acts both as a tension and as a compression spring. One end of this vibrating spring is attached to the plunger end at the magnet side and the other end is attached to the inner pole shoe. For the minimization of the transverse forces the end of the vibrating coil is attached to axially arranged flexural bars by means of specially designed mounts, the vibrating spring being retained in an intermediate piece having some play and is soldered in this position. In this manner it is admittedly possible to keep the occurring transverse forces relatively small, but on the one hand the assembly of the arrangement is relatively complicated and, on the other hand, a deterioration of the spring quality as a result of the soldering operation is unavoidable. It is the object of the present invention to improve the device of the aforementioned type in such a manner that the occurring tilting moments leading to increased wear and increased energy consumption are reduced. At the same time it is an object of the invention to reduce the space requirements of the arrangement. The reduction of the tilting moments is of great importance with respect to the drive power demand because vibrating plunger or plunger compressors of this type are in many cases not supplied by the lighting or power mains, the energy consumption being of minor importance only, but systems of this type are frequently supplied by an accumulator having, of course, a limited capacity. Plunger compressors of this type are used as refrigerant compressors in small-scale cooling plants, in particular in mobile small-scale cooling plants.